A strategy to reduce fraction number in peripheral lung stereotactic ablative body radiotherapy

Hypo-fractionated lung Stereotactic Ablative Body Radiotherapy (SABR) has often been avoided when tumours are close to the chest wall. Our strategic objective was the reduction of fraction number, while maintaining target biological effective dose coverage without increasing chest wall toxicity (CWT) predictors.

Twenty previously treated lung SABR patients were stratified into four cohorts according to distance from PTV to the chest wall, <1 cm, <0.5 cm, overlapping up to 0.5 cm and 1.0 cm. For each patient, four plans were created; a chest wall optimised plan for 54 Gy in 3 fractions, the clinical plan re-prescribed for 55 Gy in 5, 48 Gy in 3 and 45 Gy in 3 fractions.

For a PTV distance of 0.5-0.0 cm, a reduction of the median (range) D_max from 55.7 (57.5-54.1) Gy to 40.0 (37.1-42.0 Gy) Gy was observed for the chest wall optimised plans. The median V_30Gy decreased from 18.9 (9.7-25.6) cm³ to 3.1 (1.8-4.5) cm³. For a PTV overlap of up to 0.5 cm, the D_max reduced from 66.5 (64.1-70) Gy to 53.2 (50.6-55.1) Gy. The V_30Gy decreased from 21.5 (16.5-29.5) cm³ to 14.9 (11.3-20.2) cm³. For the cohort with up to 1.0 cm overlap, there was a reduction in D_max values of 9.9 Gy. The V_30Gy for clinical plans, at 66.8 (18.7-188.8) cm³, decreased to 55.3 (15.5-149) cm³.

When PTVs are within 0.5 cm of chest wall, lung SABR dose heterogeneity can be used to reduce fraction number without increasing CWT predictors.